1. Field of the Invention
This invention relates to a system and a process for absorbing electromagnetic waves over a broad frequency range. More particularly, it relates to such a system and process which is effective over a range of frequencies of from about 100 megahertz (MHz) through 10 gigahertz (GHz). Most especially, it relates to such a system and process which is effective at the very high frequency (VHF) range.
2. Description of the Prior Art
A remarkable property of ionized gas is its ability to attenuate electromagnetic waves. This wave absorption is broadband if the gas exists at atmospheric pressure. Basic theoretical studies predicting broadband electromagnetic absorption have already been done. For example, M. Mitchener and C. Kruger, Partially Ionized Gases, Chapter III, (Wiley, 1973) predict broadband electromagnetic absorption for a cold collisional plasma. W. G. Chesnut, "Radar Reflection Coefficients From a Plasma Gradient With Collisions" , pp. 96, Special Report 10, prepared for DASA, Contract DA-49-146-XZ-184, (October, 1968), discloses that an electron density that varies smoothly is necessary to maximize absorption and minimize radiation backscatter. K. G. Budden, The Propagation of Radio Waves, Chapter 15 (Cambridge, 1985), provides theoretical models for estimating the power reflection coefficient from collisional plasma gradients.
Observations of naturally occurring collisional plasmas verify the theoretical predictions. For example, N. C. Gerson, Radio Wave Absorption in the Ionosphere, pp. 379, (Pergamon Press, 1962) discloses D-layer deviative and non-deviative absorption of high frequency (HF) electromagnetic waves. S. Glasstone and P. J. Dolan, The Effects of Nuclear Weapons, Chapter X, (U.S. DOD and ERDA, 1977), discloses radio and radar interference effects of collisional plasmas produced by the detonation of nuclear explosive devices in the atmosphere. M. Gunar and R. Mennella, "Signature Studies for a Re-Entry System," Proceedings of the Second Space Congress--New Dimensions in Space Technology, pp. 515-548, Canaveral Council of Technical Societies, (April, 1965), discloses fluctuations in re-entry vehicle RCS at 60,000 feet and the communications blackout of these vehicles during re-entry due to a collisional plasma formed around the vehicles.
Some studies have been carried out specifically with helium plasmas. Y Itikawa, "Effective Collision Frequency of Electrons in Gases," The Physics of Fluids, vol. 16, No. 6, pp. 831-835, (June, 1973), provides estimates of helium momentum-transfer collision rates. Note that there is a typographical error in Table II, 10.sup.-18 should be 10.sup.-8 sec.sup.-1 cm.sup.3. R. Deloche, R. P. Monchicourt, M. Cheret and F. Lambert, "High Pressure Helium Afterglow at Room Temperature," Physical Review A, Vol. 13, No. 3, pp 1140-1176, (March, 1976), discuss theoretical and experimental investigations of the helium recombination process.
While a substantial amount of study has thus been carried out with collisional plasmas, including their property of attenuating electromagnetic waves, practical use of this phenomenon has not occurred.
Anechoic chambers conventionally employ projecting bodies of foam absorbing material mounted on the walls of such chambers. These absorbing materials are effective for absorbing higher frequency electromagnetic waves, such as in the UHF range, but they are less effective for lower frequencies, such as the VHF range. For effective absorption, lower frequencies would require projections of the foam absorbing material which are too large for ready mounting on the walls of the chamber.